Heterogeneous contact catalysts for the flameless combustion of hydrocarbons are widely used wherever substantially complete oxidation of hydrocarbons such as methane, propane and butane are desired, and these hydrocarbons are to be transformed predominantly into mixtures of carbon dioxide and water with a high thermal yield. Typical of the applications of such catalysts is in space heaters, e.g. for recreational vehicles, where ventilation is limited and wherever the actual formation of a flame may pose a problem.
The catalyst mass for such systems generally comprises a catalyser and a support; the support, being capable of withstanding the elevated temperatures at which the fuel-oxygen reaction occurs at the catalyst surface, serves as a distributor for the fuel so that the latter is reasonably uniformly spread into contact with the catalysing surface, mechanically retains the catalysing substances, provides the relatively high surface-area-to-mass or surface-area-to-volume ratios which are required and even in some cases participates in the contact-catalyst action.
It is desirable, where such heterogeneous catalysts are to be utilized in space heating apparatus for producing thermal energy by the flameless oxidation of hydrocarbons, that the reaction take place at relatively low temperatures such that the catalytic effect of the catalysing substances is not adversely affected and the useful life of the catalyst, and the apparatus in which it is incorporated, are prolonged. The most effective catalyser for the purposes described is platinum in a spongy state and spongy platinum has been applied successfully on a large number of supports. Palladium, in a finely divided state, has also been found to be an effective catalyser, also, as applied to a variety of supports.
Since both palladium and platinum are extremely expensive, it has also been proposed to form the catalyser at least in part of substances which are less expensive than these noble metals. For example, one can provide a catalyst mass which includes oxides of elements such as cobalt, cerium, chromium or aluminum, which are activated by or are employed in association with platinum and/or palladium.
Successful catalysers have included:
Platinum-alumina (Al.sub.2 O.sub.3), PA1 platinum-chromium oxide (Cr.sub.2 O.sub.3) and cobalt oxide (Co.sub.2 O.sub.3), PA1 palladium-cerium oxide (CeO.sub.2), PA1 platinum-cerium oxide (CeO.sub.2), and PA1 platinum-chromium oxide (Cr.sub.2 O.sub.3).
These catalysts provide good conversions, (i.e. percent hydrocarbons fully transformed into combustion products) when deposited upon a compact support such as upon a layer or bed of silico-alumina fibers.
They are capable of effecting an oxidation of hydrocarbons such as methane, propane, butane or heptane under conditions such that the conversion approaches 100%.
When such catalyst systems are used in heating apparatus or in other systems for the production of thermal energy by catalytic combustion, the catalyst mass is brought to the minimum temperature for a sustained reaction by the ignition of an initial quantity of gas at the surface. This initial flame combustion results in a heating of the catalyst mass to the minimum temperature for sustained flameless combustion, and the temperature is spread throughout the body of the catalyst so that, thereafter, flameless combustion is carried out across the catalyst body.
In other words, while the catalyst mass is being brought to the activation temperatures of the flameless combuster, this reaction develops locally and gradually spreads over the entire active surface of the catalyst mass, the gas being fed continuously to the latter.
While the aforementioned system is comparatively simple and allows catalytic flameless combustion systems to be utilized economically with little skill, there are some disadvantages.
One of these is the inevitable release in existing catalytic heaters of greater or lesser quantities of the uncombusted fuel gas during ignition and as the mass is brought up to temperature.
The volume of the noncombusted fuel gas which is passed or released in a function of the nature of the catalyst support or the catalyst mass itself as well as of the hydrocarbon constituting the fuel.
For example, some catalyst masses result in the generation of alkenes and like unsaturated hydrocarbons such as butene, during the priming or starting of the reaction with saturated hydrocarbon fuels.
Experience has shown that compact texture supports, such as asbestos or silico-alumina fibers, promote release of the noncombusted hydrocarbons into the atmosphere.